1. Field of the Invention
The invention relates to a method for determining the direction of the Earth's magnetic field, which may be interfered with by magnetic materials built into equipment and by magnetic fields produced by electric currents, using an electronic magnetic compass which contains three magnetic field sensors and two devices for measuring inclination.
2. Description of the Related Art
U.S. Pat. No. 4,686,772 discloses an electronic magnetic compass which, for example, is intended to be used to determine the heading of a tank. The tank has two iron bodies, namely the turret and the vehicle body, which can rotate relative to one another about a vertical axis. The magnetic compass is arranged in the vehicle body and comprises a non-pendulous triaxial magnetometer which outputs electrical magnetic field signals that represent the three magnetic field components at the site of the magnetometer. Two inclination sensors are provided, which output electrical signals that represent the pitch angle and the roll angle of the vehicle body to which the inclination sensors are fitted. Further, a device for measuring angles provides a rotational angle signal for the angle between the two iron bodies. A plurality of precalibrated correction factors are stored in a memory, these being intended to correct the effect of the magnetic field induced by the vehicle on the measurements by the magnetometer for a plurality of alies of rotation between the two iron bodies. The heading of the tank is calculated in real time using a computer on the basis of the electrical magnetic field signals and the signals from the inclination sensors, which have been corrected by the stored correction factors for the corresponding angle of rotation.
Before the described magnetic compass can be used, it is necessary to carry out calibration measurements. To do this, the vehicle is arranged on at least two different planes which are not parallel to one another. In these situations, measurements of the azimuthal angle, the pitch angle and the roll angle are taken for different orientations of the vehicle body but the same relative orientation of the tank turret. In order to make it possible to measure the last angle, a theodolite, for example, is necessary. During the calibration, it is assumed that the Earth's magnetic field at the site of the measurement is known. To do this, the values of the magnetic inclination and declination of the Earth's magnetic field for the respective measurement site are taken from cartographic sources.
U.S. Pat. No. 4,539,760 describes an electronic magnetic compass for vehicles, which has three magnetic sensors. They respond to the three orthogonal components of a magnetic field which includes the Earth's magnetic field and an additional spurious field connected with the vehicle. The magnetic sensors produce electrical signals corresponding to the components. Further, inclination sensors respond to the inclination of the vehicle with respect to the horizontal plane. A data processing unit and a memory are used to store signals which are derived as calibration correction values from the measuring sensors when the vehicle is rotated through a circle in order to calibrate the magnetic compass. In order to exclude the effect of the magnetic spurious field, the data processing unit calculates corrected values for the Earth's magnetic field at the site of the vehicle, after the calibration process has been completed and by taking the calibration correction values into account. The azimuth of the direction of travel of the vehicle is then calculated on the basis of these corrected values, using the values from the inclination sensors which are used to determine the horizon. During the calculations, it is assumed that the correction matrix is symmetric. This is true only in the rarest of cases, if ever.